1. Field of the Invention
The present invention relates to a method and apparatus for enabling circuit switched domain services to be used with Long Term Evolution (LTE) mobile devices.
2. Description of the Related Art
Third Generation Partnership Project (3GPP) is in the process of standardizing the UMTS Terrestrial Radio Access Network (UTRAN) Long Term Evolution (LTE) and System Architecture Evolution (SAE). LTE focuses on enhancement of Universal Terrestrial Radio Access (UTRA) and optimization of the UTRAN architecture. Work on the SAE defines an evolved packet core network.
IP Multimedia Subsystem (IMS) is a service platform for operators to support IP multimedia applications. Potential applications include video sharing, Voice over IP (VoIP), streaming video and interactive gaming. IMS enables mixed and dynamic services. For example, a user could be on a voice call but suddenly want to enable a video connection or transfer files. IMS by itself does not provide all of the necessary applications. Rather, it provides a framework of application servers, subscriber databases and gateways to make them possible. The core networking protocol used within IMS is Session Initiation Protocol (SIP), which includes Session Description Protocol (SDP) used to convey configuration information such as supported voice codecs. Other protocols include Real Time Transport Protocol (RTP) and Real Time Streaming Protocol (RTSP) for transporting actual sessions.
A circuit-switched network is a type of network in which a physical path is obtained for and dedicated to a single connection between two end-points in the network for the duration of the connection. A virtual circuit-switched connection is a dedicated logical connection that allows sharing of the physical path among multiple virtual circuit connections.
One of the design targets of an LTE/SAE system is cost optimization, wherein complexity is reduced by focusing on only Internet Protocol (IP) capability. In this system, the circuit switched domain is not supported when a user equipment is connected for LTE/SAE access. It is assumed that all the communication services implemented on top of IP bearers are controlled by the IP multimedia subsystem and by the respective IMS application servers. Therefore, in the LTE/SAE system, the IP multimedia subsystem is needed even if the LTE user equipment is making a simple voice call. As such, an operator must re-implement exactly the same services on the IMS machinery that are used by subscribers in a circuit switched domain in order to offer smooth migration from Second Generation (2G)/Third Generation (3G) systems to future LTE/SAE systems.
Operators require that handovers and service continuity is guaranteed when a subscriber moves across an LTE coverage border. WCDMA and/or GSM are expected to have wider coverage areas than LTE. Currently 3GPP focuses on voice call continuity procedures where a voice call is handed over between packet switched and circuit switched domains. Voice call continuity procedures on LTE/SAE systems are likely to be very complex, thus resulting in challenges associated with seamless and reliable handovers.
It is common to use IMS and SIP based call control for LTE access or other types of IP only access. However, SIP based call control does not offer the same services, for example some supplementary services, as currently implemented and used in circuit switch domains. Voice call continuity has been discussed in 3GPP and several alternatives have been proposed. There are proposals that integrate a mobile switching center (MSC) server tightly to LTE/SAE access in order to improve the voice call continuity handover. However, one thing that is common in all of the current proposals is that the actual call control in LTE/SAE access is always SIP and IMS based, which has been noted above to be inadequate.